Since the advent of transparent plastic film, such plastic has been used as protection or covers for delicate seedlings or plants particularly in cold climates. Various supports, particularly metal or wood frames, have been used with such plastic to form rudimentary greenhouse-type structures. A typical structure is shown in U.S. Pat. No. 2,889,664 issued Jun. 9, 1959. More recently a portable greenhouse having a specific frame arrangement has been described in U.S. Pat. No. 5,179,798 issued Jan. 19, 1993.
Specific plastic sheet materials have been designed for plant protection e.g. a stable woven fabric made from polyolefin flat fibers used to cover seedlings (German (East) Patent No. DD 220490 A1 issued Dec. 23, 1983). Plant protective sheets having fixed protruding microperforations are described in U.S. Pat. No. 4,798,023 issued Jan. 17, 1989. A greenhouse structure can be formed using an expandable laminated sheet or fabric; it is understood that the sheet or fabric material has some capacity to expand or shrink in response to humidity and temperature changes (see Japanese Patent No. J 61 216 622 A issued Mar. 20, 1985). In U.S. Pat. No. 5,070,643 issued Dec. 10, 1991, C. Hinsperger has described a method of improving soil conditions or grass by covering the soil or grass directly with an open-weave scrim material formed from loosely woven strips of clear plastic, the open weave permitting passage of air and moisture.
It would be desirable to provide a vented greenhouse structure wherein the effective open area of the vent varies according to temperature and air pressure inside the greenhouse.
According to this invention, it has been found possible to use a specific type of open-weave material at vent locations in a greenhouse, such that the effective open area or porosity of the material self-adjusts (increases) in response to solar thermal build-up in the greenhouse.